The present invention concerns an improvement in a frequency converter wherein the number of input/output terminals of the frequency converter can be decreased.
In prior art frequency converters, an intermediate frequency input terminal, a signal output terminal, and a local signal input terminal are provided separately so that when a number of converters are desired to be switched over, external wiring becomes extremely complicated, making it very difficult to make an arrangement incorporating the frequency converters small and compact.
A conventional type frequency converter, for instance, an up-converter as shown in FIG. 1 which uses a field effect transistor or FET 4 as a frequency converting element has a separate intermediate frequency input terminal 1 (hereinafter referred to as IF input terminal), local signal input terminal 2 and signal output terminal 3. An intermediate frequency applied to the IF input terminal 1 is sent to the source (or drain) of FET 4 via a low-pass filter (hereinafter referred to as LPF) 5 which passes the intermediate frequency but does not pass the local signal and signal output. This intermediate frequency and the frequency of a local signal supplied to the gate of FET 4 are mixed to give a signal output of a desired frequency to the source (or drain) of FET 4. This signal output is taken out of the signal output terminal 3 via a band pass filter (hereinafter referred to as BPF) 6 which passes the signal output frequency but does not pass the intermediate frequency and local signals. In this manner, the up-converter of FIG. 1 mixes the IF signal and the local signal by utilizing the non-linear characteristics of FET 4, and delivers out a desired side band at the signal output terminal 3 through BPF 6.
Another conventional up-converter shown in FIG. 2 provides as FET 4 with a feedback circuit 7 to constitute an oscillator which oscillates at a local frequency, thereby eliminating the local signal input termimal. The up-converter of FIG. 2 operates similarly to that shown in FIG. 1.
These up-converters require two or three input/output terminals. For instance, if several up-converters shown in FIG. 2 are to be used by switching them over, numerous external wiring conductors are necessary as shown in FIG. 3 and the wiring becomes quite complicated. More particularly, in an arrangement using four up-converters 30-1, 30-2, 30-3 and 30-4, a two-stage, four-contact transfer switch 8 is provided. Each of the stationary contacts on the first stage 8a are connected to IF input terminal 1 of the respective up-converters while the movable contact is connected to a terminal 9 which constantly receives the intermediate frequency. On the other hand, the respective stationary contacts on the second stage 8b are connected to the signal output terminal 3, and the movable contact is connected to a terminal 10 from which the output is fed out. Accordingly, switchover of the transfer switch 8 will activate one of the up-converters 30-1, 30-2, 30-3, and 30-4 to allow taking out a desired signal output from the terminal 10. In this example, 8 external wiring conductors are required between the terminals 1, 3 and the switch 8, and when the number of up-converters is further increased, or when the local signal input terminal 2 is provided as in the case of the up-converter shown in FIG. 1, the external wiring problem is aggravated, becoming bulky and increasingly complicated, thus presenting obstacles in making the arrangement small and compact.